The design of an ignition system for a solid propellant rocket motor generally requires that an adequate pressure build up take place in a metal component for the igniter mix to be injected into the propellant's perforation to accomplish ignition. An effect associated with rocket motor ignition is termed: "ignition-induced pressure waves". This effect is more prevalent when ignition takes place at a rate which is less than that rate required for simultaneously and uniformly igniting the propellant.
Most propellant grains are readily ignited by a typical igniter or squib. The typical igniter or squib is comprised of the basic elements which include a bridgewire, an explosive charge contained in a cup which has an open or an end which contains an easily ruptured baffle which is directed towards the propellant grain, and lead wires which are encased in a sealer plug for the cup. An electrical source is connected to the lead wires which supplies the proper amperes to the bridgewire for firing.
Variations of propellant igniters or squibs have been designed to meet the requirements for a particular rocket motor. For example, one squib is comprised of pyrotechnic black powder encased in a thin plastic cup with the open end directed towards the propellant grain for aft-end ignition. This type igniter is referred to as an aft-end packaged igniter which is used on the TOW missile. Other igniter types include: a jelly-roll or tubular igniter, as used in Little John; a head-end packaged igniter, as used on various spin motors; and a head-end pyrogenic igniter, as used on the HERCULES sustainer.
Transient analysis of typical advanced interceptor motors indicated that conventional ignition systems, such as, pyrogens and pyrotechnic igniters could be used to ignite low-altitude commit, interceptor-type, first-stage motors and gain up to 75% of maximum pressure in less than 0.05 second. However, the very high acceleration forces acting on the second-stage makes the use of a grain support tube necessary, and this means that the conventional ignition systems cannot be used for igniting the second stage motor since it would prevent the igniter gases from impinging on the propellant's surface. The requirements for a grain support tube was verified in advanced interceptor propulsion subsystem design studies. The design studies confirmed that a grain support tube would be necessary to prevent the second-stage propellant grain from being extruded out the nozzle during first-stage operation. It must be emphasized that this support tube needs to be perforated so that it can remain in place permanently. As a result, when the propellant undergoes ignition, its exhaust gases are ported through the perforations in the support tube, and then flow down the perforation on the inside of the support tube and inside the propellant grain. There have been similar methods for reinforcing the propellant grain developed, such as, foamed mandrels, frangible mandrels, etc. These were mainly intended to eliminate the need for the casting mandrel which would not have to be pulled after the propellant had been cured. This type of mandrel would fill the entire propellant's perforation, be left in place, and then blown out on ignition of the grain. The primary purpose of these mandrels was to reduce the cost of rocket motor manufacture, and would be particularly practical for small motors which would be produced in volume quantities.
Thus a supporting tube or a supporting mandrel is one way of meeting the requirements for offsetting the effect of high axial acceleration; however, this is not to be confused also with the use and functions of prior art propellant traps for rocket motors. The use of traps to merely retain propellants within the combustion chamber of a rocket motor is well known. For example, as disclosed in U.S. Pat. No. 2,605,607 issued to C. N. Hickman on Aug. 5, l952, traps were used to retain sticks of powder employed as propellant in rocket motor combustion chamber. Without the traps to retain the propellant, pieces of propellant would be discharged out the nozzle and would result in loss of total thrust, wide pressure variations out the nozzle, and possibly failure of the rocket motor if restriction of the nozzle became too great. Ignition of the sticks of powder was accomplished by an igniting element mounted within the interior of the trap member.
The present invention relates to a method of ignition of the solid propellant grain in the second stage motor of an advanced interceptor. The advanced interceptor has required the development of propellant grains having ultrahigh burning rates. One way to achieve ultrahigh burning rates, other than by modification of the chemical composition of the propellant, is to design grains having the desired web thickness and with a proper design of the central perforation to achieve the desired burning pattern and rates. However, the described combination has resulted in a propellant having inadequate mechanical properties unless compensated for by employing a support tube for the propellant grain. The high acceleration rates of the interceptor creates problems because of the inadequacy of the mechanical properties of the propellant. The unsupported propellant grain tends to flow, and break-up of the propellant grain occurs under acceleration loads, especially before the motor is pressurized--a condition which exists during the period when the first stage or booster of the interceptor is operating. A perforated support tube is the answer to the problem of providing reinforcement to the propellant grain, and thus, serves to prevent propellant flow and propellant break-up. However, the use of such a support tube rules out the conventional methods of igniting solid propellant grains.
Therefore, an object of this invention is to provide a method of igniting a solid propellant grain that employs a perforated support tube or a consumable mandrel for supporting the grain.
Another object of this invention is to provide a method for igniting a perforated tube supported propellant grain to accomplish a more simultaneously and uniformly igniting of the propellant grain to thereby eliminate the ignition-induced pressure waves.
A further object of this invention is to provide an ignition system for igniting a perforated tube supported propellant grain which provides greater control and ease of manufacture and predictability of the ignition system.